Minor missing functionality added:

• enum: add missing Enum.value property. #2739
• Allow thread termination to be avoided during shutdown for CPython 3.7+ via .disarm for gil_scoped_acquire/gil_scoped_release. #2657

Fixed or improved behavior in a few special cases:

• Fix bug where the constructor of object subclasses would not throw on being passed a Python object of the wrong type. #2701
• The type_caster for integers does not convert Python objects with __int__ anymore with noconvert or during the first round of trying overloads. #2698
• When casting to a C++ integer, __index__ is always called and not considered as conversion, consistent with Python 3.8+. #2801

Build improvements:

• Setup helpers: extra_compile_args and extra_link_args automatically set by Pybind11Extension are now prepended, which allows them to be overridden by user-set extra_compile_args and extra_link_args. #2808
• Setup helpers: Don't trigger unused parameter warning. #2735
• CMake: Support running with --warn-uninitialized active. #2806
• CMake: Avoid error if included from two submodule directories. #2804
• CMake: Fix STATIC / SHARED being ignored in FindPython mode. #2796
• CMake: Respect the setting for CMAKE_CXX_VISIBILITY_PRESET if defined. #2793
• CMake: Fix issue with FindPython2/FindPython3 not working with pybind11::embed. #2662
• CMake: mixing local and installed pybind11's would prioritize the installed one over the local one (regression in 2.6.0). #2716

Bug fixes:

• Fixed segfault in multithreaded environments when using scoped_ostream_redirect. #2675
• Leave docstring unset when all docstring-related options are disabled, rather than set an empty string. #2745
• The module key in builtins that pybind11 uses to store its internals changed from std::string to a python str type (more natural on Python 2, no change on Python 3). #2814
• Fixed assertion error related to unhandled (later overwritten) exception in CPython 3.8 and 3.9 debug builds. #2685
• Fix py::gil_scoped_acquire assert with CPython 3.9 debug build. #2683
• Fix issue with a test failing on PyTest 6.2. #2741

Warning fixes:

• Fix warning modifying constructor parameter 'flag' that shadows a field of 'set_flag' [-Wshadow-field-in-constructor-modified]. #2780
• Suppressed some deprecation warnings about old-style __init__/__setstate__ in the tests. #2759

Valgrind work:

• Fix invalid access when calling a pybind11 __init__ on a non-pybind11 class instance. #2755
• Fixed various minor memory leaks in pybind11's test suite. #2758
• Resolved memory leak in cpp_function initialization when exceptions occurred. #2756
• Added a Valgrind build, checking for leaks and memory-related UB, to CI. #2746

Compiler support:

• Intel compiler was not activating C++14 support due to a broken define. #2679
• Support ICC and NVIDIA HPC SDK in C++17 mode. #2729
• Support Intel OneAPI compiler (ICC 20.2) and add to CI. #2573